The idiopathic inflammatory bowel diseases (IBD) which include Crohn's disease (CD) and ulcerative colitis (UC) are chronic disorders of the gastrointestinal tract of unknown etiology with a combined prevalence of about 150-200 cases per 100,000 in Western countries. CD is a multifactorial disease and thought to be caused by an abnormal inflammatory response directed against enteric microflora. A major susceptibility locus for CD was originally mapped at the pericentromeric region of chromosome 16 (IBD1 locus) and later identified as NOD2/CARD15. Three major NOD2 mutations, L1007fsinsC, R702W and G908R have been found to be associated with CD in Caucasian populations. Importantly, having one copy of the mutated alleles confers a 2-4-fold increased risk of developing CD, whereas homozygocity or compound heterozygocity for NOD2 mutations increases the risk 20-40-fold, indicating that NOD2 acts in a recessive fashion and that loss of NOD2 function is important for disease development. The three NOD2 variants are deficient in their ability to activate NFkappaB in response to bacterial PGN and synthetic MDP, the essential moiety in PGN recognized by NOD2. In preliminary studies, we find that peripheral blood mononuclear cells (PBMC) from individuals homozygous for the main L1007fsinsC mutation are defective in the response to MDP. Thus, defective sensoring of bacteria by impaired recognition of PGN may be critical in the pathogenesis of CD. Studies of CD patients have revealed that only between 10 and 20% of CD patients are homozygous for NOD2 mutations in Western populations. Thus, we hypothesize that molecular defects in the PGN signaling pathway other than that associated with NOD2 mutations may be found in at least some CD patients. We find that MDP alone is not sufficient to interact with NOD2, suggesting that cellular factors are required for the recognition of MDP and NOD2 activation. To address this hypotheses, we propose three specific Aims to (i) determine the response of PBMC and intestinal macrophages from CD patients and controls to MDP and relate the findings to NOD2 genotype (ii) characterize the mechanism of NOD2 activation in response to MDP and (iii) identify and characterize cellular factors involved in the regulation of NOD2 and recognition of MPD by NOD2. The proposed studies may provide novel insight into the role of NOD2 in the pathogenesis of CD and reveal the molecular mechanism Whereby NOD2 is activated in response to bacteria. In addition, our studies may identify novel genes involved in the NOD2 signaling pathway regulating the susceptibility to CD.